Head injury in humans is often complicated by secondary hypoxia- ischemia occurring days after trauma. The central hypothesis of the present proposal is that the brain remains extremely vulnerable to secondary hypoxia for several days after traumatic injury. The objectives of this project are to identify regions injured by secondary hypoxia 24-72 hours after traumatic brain injury (TBI) and to investigate the mechanisms of increased vulnerability. In addition,, methods for monitoring secondary hypoxic injury in vivo will be evaluated. Four specific aims are proposed. First, we will determine whether sublethal hypoxia administered 24 or 72 hours after TBI augments injury in the cerebral cortex. We will test the hypothesis that hypoxia-induced cortical spreading depression (CSD) enlarges the cortical lesion by precipitating energy failure in the traumatic penumbra. Second, we will determine whether secondary hypoxia at these times exacerbates traumatic injury in the white matter. We will test the hypothesis that the blood flow response to hypoxia is impaired in traumatized white matter, causing energy failure calpain activation, and oligodendroglial apoptosis. Third, we will determine whether hippocampal injury is worsened by secondary hypoxia at 24 or 72 hours after TBI. We will test the hypothesis that trauma lovers the hypoxic threshold of the mitochondrial release of cytochrome c, thus promoting apoptosis. Fourth, we will determine whether sublethal hypoxia diminishes evoked potentials and tissue pO2 to a greater extent at 24 or 72 hour after TBI than in uninjured brain. We will test the hypothesis that TBI alters the threshold for loss of function and O2 delivery and during secondary hypoxia. Injury in the cerebral cortex, white matter, and hippocampus will be assessed using histopathology, immunocytochemical markers for calpain and case activation, and TUNEL staining. Regional blood flow methods, microanalysis for energy metabolites, and Western blots for cytochrome c will be employed to evaluate vascular and mitochondrial functions. The experiments will be performed using lateral fluid-percussion brain injury in rats as the primary experimental model, but will also be extended to models of predominantly gray matter or white matter injury. The results of the proposed experiments should identify regions vulnerable to secondary hypoxia after TBI, indicate mechanisms of vulnerability, and underscore the need for aggressive monitoring and treatment of secondary hypoxia- ischemia in human head injury.